"Bird bombs" and propane cannons, whose targets are the ears of 10,000 blackbirds, blasted away yesterday for the second straight evening in an attempt to scare off the flock of birds that has befouled a Herndon neighborhood for more than a month.

Although Fairfax County environmental officials said the birds have not yet caused a health hazard, the town of Herndon continued with its noisy battle against them. The birds turn the sky black when they fly and the ground below smelly when they perch, residents of the town complain.

"You're going to have to keep it up for two or three nights," Herndon officials were told yesterday by a state bird control expert. "If the birds fly in and don't see or hear their buddies, they'll leave," said William Philip Eggborn, of the Virginia Department of Agriculture and Commerce.

"If you don't see any birds Thursday or Friday, you can safely quit," Eggborn said.

Shortly before dusk on Tuesday evening and again last night, Herndon police took borrowed bird-scaring weapons obtained from Montgomery County and started shooting. They shot bird bombs, which are fired from .22 caliber pistols making loud popping noises. They fired propane cannons, which are tubes in which igniting gas makes a crackling blasts.

About 60 percent of the birds were scared away Tuesday night according to a police spokesman. Scaring them does not mean they will not come back, Eggborn said.

Herndon officials said they may also seek a more permanent solution to the bird problem by asking owners of wooded land, where the birds are roosting between Arkansas and Eldin Avenues to thin their trees or face action under the town code.

Neither state nor county laws can force property owners to alter their land unless it is declared a health hazard, according to county officials.

The birds would have to return for three or four years and build up at least an inch of droppings before they could be considered a health hazard, according to James F. Phillips, senior environmental health sanitarian for Fairfax County.

Then, Phillips said, a fungus develops in the ground called histoplasmosis. It's spores cause an airborne disease resembling tuberculosis that caused respiratory problems in children. The disease was common in chicken houses years ago, Phillips said, but is rare today.

A more likely problem resulting from the birds, Phillips said, would be an infestation of rats if the droppings continue.

Meanwhile residents will just have to put up with the sour smell and occasional dead birds in their lawns, officials agreed.Blackbirds have a life expectancy of one year so its is not unusual to find dead birds in the area where they roost, Phillips said.